1. Field of the Invention
The present invention relates to fiber optic transmission systems and dispersion compensating circuits associated with optical transmission systems. In particular the present invention compensates for the laser chirp and chromatic dispersion distortions to enable effective broadband transmission and extended fiber link reach.
2. Background of the Invention
Optical transmission systems are widely used to transmit data on a broadband network. In a typical optical transmission system, a laser provides an optical signal at a predetermined frequency, which is typically modulated to provide an optical transmission data signal.
In Broadband Passive Optical Network (B-PON) or Gigabit-Passive Optical Network (G-PON) applications, the longest distance is 20 km. So the dispersion compensation needs only to be adequate for fiber lengths up to 20 km.
In Hybrid Fiber Coaxial (HFC) networks and narrowcast overlay digital applications, where analog and digital channels are combined at the HFC hub site, operators are requesting Dense Wavelength Division Multiplexing (DWDM) narrowcast transmitters, to transmit at higher optical power, e.g., 10 dBm, and to carry wider bandwidth of digital payload for longer reach, e.g., up to 100 KM of single mode fiber. Extending the link reach will produce Low Frequency Noise Rise (LFNR) due to fiber dispersion in the analog channels band.
Normally a 1550 nm (nanometer) optical signal is assigned for video signal transmission. Typically, an externally modulated laser is selected as the optical source because it has much lower laser chirp than a directly modulated laser transmitter. Laser chirp is the shift in the laser output wavelength/frequency resulting from the modulating signal. A directly modulated laser transmitter, especially a high power laser transmitter, may have a total laser chirp up to several GHz because of the large laser modulation Radio Frequency (RF) current. The large total laser chirp helps improve the Stimulated Brillouin Scattering (SBS) suppression optical power level, but as a result of fiber dispersion, introduces serious second order distortions, such as Composite Second Order (CSO) distortion.
When the chirped optical frequencies pass through a fiber, different optical wavelengths propagate at different group velocities, which causes delay dispersion at the receiving end and often causes distortions in the communication signal. CSO distortions often occur in the low −40 dBc range. For the B-PON or G-PON laser transmitter to be effective, the laser transmitter CSO distortions should be better than −60 dBc. A narrowcast laser transmitter, which transmits for example 100 MHz of digital payload of 256 QAM, and depending on the laser chirp and fiber link reach, could produce more than 5 dB of LFNR in the analog channels frequency domain, which greatly degrades the transmission of analog channels in case of a narrowcast overlay application. Accordingly, a high degree of CSO correction ability in the high frequencies and very accurate adjustable distortion compensation is needed in order to use a directly modulated laser.
The problem of fiber dispersion compensation has been investigated in great detail and various techniques have been used to solve this problem. The solutions were in both optical domain and electronic domain.
Dispersion compensation fiber (DCF) or chirp fiber Bragg grating (CFBG) can be used for the compensation devices in the optical domain. DCF is an optical fiber that has exactly the opposite dispersion effect as a regular single mode fiber. CFBG is a component that reverses the group delay compared to the ordinary fiber group delay between wavelengths.
The advantage of optical technique is its precision. However, the DCF is costly, adds attenuation, needs additional amplifications and is difficult to adjust/readjust. The CFBG optical attenuation is low, but optical bandwidth is limited to about one nm, thus reducing laser source choices and introducing the need to stabilize laser wavelength.
Electronic compensation techniques are significantly more cost effective. In the electronic domain, pre-chirp compensation techniques have been widely used in digital applications.
One method is described in U.S. Pat. No. 5,115,440 to Hermann Gysel, et al. In this patent, a single varactor-tuned delay line network is inserted between the modulating signal source and the laser. A varactor is a type of diode designed to function as a variable capacitor, the varactor's capacitance is a function of the instantaneous voltage impressed on its terminals. The delay line network provides an instant amplitude-dependent delay of the positive portion of the modulating signal applied to the laser and compensates the fiber delay caused distortion so that the CSO distortion can be reduced. This electrical compensation technique is not sensitive to the transmitted optical wavelength, it works for all 1550 Dense Wavelength Division Multiplexing (DWDM) and Coarse Wavelength Division Multiplexing (CWDM) system applications, and is easy to adjust electronically.
This approach worked very well for low optical power (1-3 mw) lasers with relatively large laser chirp (1.8 Ghz/ma). However, modern 1550 nm lasers have much lower laser chirp and may have much larger optical power, e.g., laser chirp now may be between 0.03-0.1 Ghz/ma and optical power may up to 10-20 mw, i.e., 10-13 dBm.
Large optical power is important for B-PON and G-PON applications as well as for narrowcast transmitters in HFC networks. A large optical output power laser usually has more total laser chirp, so the SBS suppression optical power level will be larger. Further, when light having a larger optical power transmits through an Erbium Doped Fiber Amplifier (EDFA), it improves the systems' signal-to-noise ratio.
A directly modulated laser transmitter has advantages for use in a B-PON or G-PON system, if CSO distortion problems can be eliminated. Directly modulated laser transmitters are much cheaper than externally modulated laser transmitter. Reliability and temperature stability of the directly modulated laser transmitters are much better than the externally modulated laser transmitters. In the directly modulated laser transmitter, the Optical Modulation Index (OMI) is usually at least 1-2 dB higher than for an externally modulated laser transmitter. Thus the signal-to-noise ratio of the B-PON system using directly modulated laser transmitter can be 1-2 dB higher. By using the directly modulated laser transmitter, due to the large laser total chirp, the SBS suppression optical power level can be higher than for the externally modulated laser transmitter. This is very useful for B-PON and G-PON applications.
Reducing and eliminating LFNR in DWDM narrowcast transmitters is important in the transmission success of DWDM narrowcast overlay applications.
Large optical power usually requires larger RF drive voltages. For example, for most 1550 nm lasers with a power range from 10-13 dBm, the peak RF drive voltage will be 4-8 volts. For older, low optical power lasers, peak RF driving voltages were less than one volt. Driving the varactor with large RF voltages greatly increases the nonlinearity of the capacitance change with voltage. Due to the large RF driving voltage, compared to the prior art, the RF driving voltage needs to be pre-distorted in order to provide a linear change in capacitance. Also, a very smooth control of dispersion compensation method is needed for CSO correction.
What is needed is a varactor network for distortion compensation to be used with 1550 nm lasers that have a chirp between 0.03-0.1 Ghz/ma and optical power up to 10-13 dBm.